In a wafer fabrication facility, optical scanning is one of the recognized methods for inspecting the wafer. The scanning irradiates a specific region of the wafer, for instance in a die or cell on the wafer, and measures one or more parameters of the returning radiation. The measured parameters may be compared with other respective, assumed “standard” parameters, typically in a cell-cell or die-die comparison, or in a comparison against previously determined values, to determine if the irradiated region is within specification.
In all cases, the comparison relies on knowledge of the locations both of the region being inspected, as well as the region from which the standardized parameters have been taken.
Two trends in wafer fabrication increase the difficulty of locating a specific region within a wafer. A first trend is the increase in overall wafer size; a second trend is the reduction in size of elements within the wafer. Wafer sizes have increased from initial diameters of 100 mm to the 300 mm diameters typical today. Rule sizes, of elements on the wafer, have decreased from initial values of more than 1 micron to 40 nm. Wafer sizes are set to increase, and rule sizes continue to decrease.
The prior art has a number of approaches which claim to improve the process of locating regions.
U.S. Patent Application 20040066506 to Elichai et al. whose disclosure is incorporated herein by reference, describes a system for compensating for inaccuracies in an optical scanner used in a surface inspection system. A surface of an article is scanned along a scanning axis and a scanning axis signal is output at predetermined distances along this axis. A jitter signal is output whenever the scanner deviates from the scanning axis, and is used to calculate the amount of deviation.
U.S. Pat. No. 5,825,482 to Nikoonahad, et al., whose disclosure is incorporated herein by reference, describes a system to correct for misregistration errors. A reference vector of data samples is obtained by averaging adjacent data sample vectors. A comparison of the data samples in a current vector being processed to data samples that may be offset from the current vector along the direction of the current vector enables the detection and correction of misregistration errors.